Support GitHub Enterprise

This issue was started by a [e-mail message by Ronnie Ridpath](http://librelist.com/browser//git.hub/2013/11/1/how-to-setup-with-github-enterprise/#4d0c4ad124b6f94b787aa2bead13c21e) sent to the mailing list.

Some steps that need to be made to make this happen:

- [x] Make `base_url` configurable
- [x] Validate `base_url` (make sure the URL is at least syntactically valid)
- [x] Make `RequestManager` able to handle a `base_url` that isn't at the root of the server (`http://example.com/api/v3` in particular)

Other changes might be needed.